Physics P3.1

X-rays are part of the electromagnetic spectrum. They are quite far up in the spectrum and therefore have a high frequency and a low wavelength.

There are good properties of X-rays, including that they affect photographic film, that they are absorbed by metal and bone, and that they are transmitted by healthy tissue.

These qualities give them the useful potential to form images of bones on photographic film which can be easily analysed for fractures and dental problems.

Charge-coupled devices (CCDs) can be used to form electronic images of X-rays.

CT scanners can produce digital images of a section of the body.

Body organs made of soft tissue can dissolve contrast medium so that X-rays are absorbed and so they can be seen.

X-rays can cause ionisation in cells and damage living tissue. Therefore, people who work with X-rays daily need to take precautions.

P3 1.2

Sound that has a higher frequency of 20 000Hz (the amount we can hear) is known as ultrasound.

Electronic systems can produce ultrasound waves. Part of a wave is reflected when a wave meets a boundary. The wave travels back through the material to a detector. The detector detects the time it took for the sound wave to travel back. From the information it gets, the electronic system can form an image.

It calculates it like so:

distance travelled in metres = speed of wave in m/s times by time in seconds

But, it is crucial to remember that the distance that it has travelled is to the boundary and back.

Ultrasound is used in medicine for scanning, because it doesn't ionise. Therefore, it cannot be harmful. Ultrasound can also be used as therapy, for example to shatter a kidney stone into pieces.

P3 1.3

Refraction is when light changes direction as it passes through a transparent boundary. Refraction occurs because of how fast light moves through different substances. However, refraction doesn't occur if the light is travelling along the normal.

From air to glass, the light moves closer to the normal. The angle between the light before refracted and the normal is called the angle of incidence. The angle between the light ray after it has been refracted and the normal is called the angle of refraction. You can work out the refractive index of a glass block or substance by using this equation: